User operated drug delivery devices are as such known in the prior art. They are typically applicable in circumstances, in which persons without formal medical training, i.e., patients, need to administer an accurate and predefined dose of a medicinal product, such as heparin or insulin. In particular, such devices have application, where a medicinal product is administered on a regular or irregular basis over a short term or long-term period.
In order to accommodate with these demands, such devices have to fulfil a number of requirements. First of all, the device must be robust in construction, yet easy to use in terms of handling and in understanding by the user of its operation and the delivery of the required dose or medicament. The dose setting must be easy and unambiguous. Where the device is to be disposable rather than reusable, the device should be inexpensive to manufacture and easy to dispose. Moreover, the device should be suitable for recycling. To meet these requirements, the number of parts required to assemble the device and the number of material types the device is made from need to be kept to a minimum.
Such pen-typed injectors are typically adapted to receive a replaceable and/or disposable cartridge containing the medicinal product to be dispensed by means of the device. The cartridge comprises an outlet to be coupled with a piercing element, e.g. an injection needle, a cannula or the like in a fluid transferring way. Further and in order to expel a predefined dose of the medicinal product, a plunger of a drug delivery device is adapted to act on the piston for displacing said piston by a predefined distance in distal, thus dose-dispensing direction.
FIG. 1 shows a cross-sectional illustration of a piston 16 slidably disposed inside a circumferential cylindrical wall 24 of a cartridge 23. The cartridge 23 is arranged inside a drug delivery device that comprises a proximal housing component 20 and a cartridge holder 22. The housing 20 accommodates a not further illustrated drive mechanism, that serves to drive a piston rod 10 and a bearing disc 12 in a distal axial direction, hence downward in the illustration of FIG. 1. For this purpose, the bearing disc 12 is rotatably mounted on a lower, hence distally located end section of the piston rod 10. The radial dimensions of the bearing disc 12 substantially match with the size of the proximal end face of the piston 16.
The piston 16 comprises two annular sealing surfaces 25 radially abutting against the inner side wall 24 of the cartridge 23. In this way, the piston 16 provides a durable and leak-proof seal for the medicinal product contained in the cartridge 23. As can be seen in the cross-section of FIG. 1 and as further illustrated in a top view illustration of FIG. 2, the thrust receiving surface comprises four rectangularly shaped distance elements, which in the course of a mass production process are adapted to prevent mutual adhering of pistons, e.g. in a feeding arrangement.
In the illustration of FIG. 1, the bearing disc 12 and the proximal end face, the thrust receiving surface of the piston 16, are not yet in mutual contact. During dose dispensing, the piston rod 10 typically becomes subject to a rotative movement. Due to a threaded engagement with a radially inwardly protruding thread 18, the piston rod 10 is displaced in distal direction when rotated. Consequently, the bearing disc 12 gets in direct contact with the proximally located thrust receiving surface of the piston 16 and in response to a further applied thrust, the piston 16 becomes displaced in distal direction, that is downwards in FIG. 1.
In practice, due to manufacturing and assembly tolerances radial position of the piston rod 10 and/or the bearing disc 12 may vary within certain limits. Hence, a piston rod 10 and/or a bearing disc 12 can be radially displaced with respect to the center of the piston 16. If not properly aligned, it may occur, that the force provided by the piston rod 10 and the bearing disc 12 is non-centrically transferred to the piston 16. Such radial offset may in turn lead to a cant or tilt of the piston 16, which is flexibly deformable to a certain extent. As a consequence, a displacement force required for distally displacing the piston 16 may substantially rise. Additionally, also the dosing accuracy may decrease when a distally directed driving force is non-centrally transferred to the piston 16.
Also, since the distance elements 14 protrude from the proximal end face of the piston 16, thrust being applied to the piston 16 is entirely received by the distance elements 14, which, as a consequence may become squeezed. However, such point stresses and squeezing effects may further have a negative impact on the dosing accuracy of the drug delivery device and its drive mechanism.